Catalysis is the basis for many industrial/commercial processes in the world today. The most important aspect of a catalyst is that it can increase the productivity, efficiency and profitability of the overall process by enhancing the speed, activity and/or selectivity of a given reaction. Many industrial/commercial processes involve reactions that are simply too slow and/or efficient to be economical without a catalyst present. For example, the process of converting natural gas or methane to liquid hydrocarbons (an extremely desirable process) necessarily involves several catalytic reactions.
The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is catalytically reformed with water to produce carbon monoxide and hydrogen (i.e., “synthesis gas” or “syngas”). In a second step, the syngas intermediate is catalytically converted to higher hydrocarbon products by processes such as the Fischer-Tropsch Synthesis. For example, fuels with boiling points in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes may be produced from the synthesis gas.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming or dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, the reaction proceeding according to Equation 1.CH4+H2⇄CO+3H2  (1)
The catalytic partial oxidation (“CPOX”) of hydrocarbons, e.g., methane or natural gas, to syngas has also been described in the literature. In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial or direct oxidation of methane yields a syngas mixture with a more preferable H2:CO ratio of 2:1, as shown in Equation 2:CH4+1/2O2⇄CO+2H2  (2)
The H2:CO ratio for this reaction is more useful for the downstream conversion of syngas to chemicals such as methanol or other fuels than is the H2:CO ratio from steam reforming. However, both reactions continue to be the focus of research in the world today.
As stated above, these reactions are catalytic reactions and the literature is replete with varying catalyst compositions. The more preferred catalytic metals are Group VIII or noble metals. These metals are sometimes combined with secondary metals to enhance their activity. Further, the catalyst compositions include particular support materials such as alumina, silica, titania, and the like.
After a period of time in operation, a catalyst will become deactivated, losing its effectiveness for enhancing the desired reaction to a degree that makes it uneconomical at best and inoperative at worst. This process is generally known as “aging.” The more aged a particular catalyst the less efficient the catalyst is at enhancing the reaction, i.e., less activity it has. At this point, the catalyst can be either replaced or regenerated. However, replacing a catalyst typically means discarding the deactivated catalyst. A discarded catalyst represents a loss of expensive metals. Alternatively, the user may send the catalyst back to the supplier for recovery of expensive metals, such as Rh, Pt, Pd, etc. However, the recovery process involves dissolving the multi-component catalyst and subsequent separation of the active components from the mixed solution. The chemistry is complex and costly, more importantly, it involves bulk amounts of harsh chemicals that ultimately must be discarded and the use of landfills for such disposal is problematic. For all of these reasons, regeneration is preferred over replacement. Regeneration generally refers to any process used to restore all or some of the original activity to the deactivated catalyst.
Catalysts systems can become deactivated by a number of processes, including coking, sintering, poisoning, oxidation, and reduction. Most of these processes are reversible, although not economically feasible. The most common deactivation mechanism is coking or fouling of the catalyst. Simply put, coking is the formation of hydrocarbonaceous residue on the surface of the catalyst. As the coke forms, it physically blocks the reactants from reaching active sites. Over time the coke will completely engulf or cover the active sites preventing the catalytic reaction from taking place. Further, the coke results in an increase in pressure drop across the catalyst bed due to the buildup of carbon in the flow paths or interstices.
Another deactivation mechanism is sintering. Sintering is particularly difficult to regenerate and has traditionally been viewed as a non-reversible phenomenon. Sintering is the process in which fine particles of a material become chemically bonded together without melting to form a mass. In the case of syngas catalysts, the fine particles of catalytic metal agglomerate together to form a large mass of catalytic metal effectively causing a decrease in the activity due to the decrease in surface area.
Because regeneration has traditionally been so difficult, the catalytic metals are typically dissolved and recaptured for use in new catalyst batches. However, research is continuing on the development of more efficient syngas catalyst systems and catalyst systems that can be more effectively regenerated. At the present time, there are no known methods that are economically feasible for regenerating a sintered syngas catalyst.
Hence, there is still a great need to identify new partial oxidation catalysts, particularly partial oxidation catalysts that are less susceptible to deactivation.